Electrical safety detonator



M. REYNE ETAL Filed June 4 1962 INVENTORS 13W 226w ATTORNEY ELECTR I CALSAFETY DETONATOR i A wwwwww mwwmn w QM M \.x\ 6 y w March 30, 1965United States Patent 9 Claims. (Cl. 102-28) The invention relates toelectrical safety detonators for use in well bores to detonateexplosives or shaped charges which provide a penetration in earthformations. In particular, this invention relates to detonators whichare useful where well bore pressures and temperatures are very high.

Perforating apparatus for use in well bores typically includes a shapedcharged device which is detonated by a blasting cord. The blasting cordis in turn detonated by a blasting cap or detonator. Detonators orblasting caps in this type of use are often exposed to the fluids in thewell bore and consequently, the pressures and temperature of the fluidin the well bore. Conventional detonators are not practical for usewhere the pressures exceed about 14,000 p.s.i. and the temperaturesexceed about 200 C. Moreover, in well bores where gas is present underconsiderable pressures, conventional detonators, which generally haverubber or neoprene seals, are not gas-tight so that their operation isfrequently faulty.

Also, detonators of known types are normally fitted with low-resistanceigniter wires to ensure their very quick ignition. In order to protectthese detonators from being prematurely detonated by stray currents andvoltages, resistors of appreciable value are coupled to the outside ofthe detonators in an electrical series connection with the ignitingwire. These additional exterior resistors are sometimes inadvertentlyshorted or opened to cause a detonator failure. Igniting compounds of avery low sensitivity are sometimes used to avoid premature detonation ofa detonator from such things as atmospheric electrical discharges. Suchigniting compounds like aluminothermic mixtures (25% Al+75% Fe O or 20%Al+80% CuO for instance) react only at temperatures from 1000 C. andrequire ignition currents of several amperes. Cables conventionally usedin well bores are not designed to carry such currents so such ignitingcompounds of low sensitivity are not usually used in well bores.Moreover, in conventional detonators it is possible to developrelatively high static charges on the detonator as a result of frictionwith the metal walls. Hence, it is possible for a spark to leap betweenthe detonator jacket or wall and one of the terminals of the heatingresistor to prematurely ignite the detonator. This, of course, can beeliminated by a high loss insulator, such as rubber of high carboncontent covering the detonator but nonetheless, the hazard is stillpresent.

It is an object of this invention to provide new and improved detonatorswhich operate satisfactorily at temperatures in excees of 200 C. and atpressures in excess of about 14,000 psi.

Another object of this invention is to provide new and improveddetonators which are hermetically sealed.

It is a further object of this invention to provide new and improveddetonators whose safety in the presence of stray currents or of staticcharges is excellent.

It is another object of this invention to provide a new and improvedsafety detonator needing a low igniting current.

Still another object of this invention is to provide new and improveddetonators which are easy and safe to use.

3,175,492 Patented Mar. 30, 1965 "ice An electrical detonator accordingto the invention, comprises a hollow metal receptacle with a suitablewall thickness. Sealed in the receptacle by a metal cover are adetonating or explosive charge, an inflaming or igniting compoundwherein the igniter is connected to at least one conductor traversingthe cover closing the receptacle. The conductor is sealed in the coverby an insulating glass bead. The igniting compound is more particularlya sensitive mixture of the aluminothermic type.

According to another feature of the invention, the elec trical igniteris made of a helically wound wire whose resistance is of some few dozenohms which prevents untimely operation of the detonator in the presenceof interference currents or charges.

The novel features of the present invention are set forth withparticularly in the appended claims. The present invention, both as toits organization and manner of operation together with further objectsand advantages thereof, may best be understood by way of illustrationand example of certain embodiments when taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a view in cross-section of the detonator casing of the presentinvention;

FIG. 2 is a view in cross section of the igniter subassembly for thedetonator; and

FIG. 3 is a view in cross section of the assembled detonator.

According to FIG. 1, a hollow, cylindrically shaped receptacle 10 isshown constructed, for example, of steel of sufficient thickness towithstand high pressures. The receptacle can, for example, be 1.97inches long with a wall 12 of 0.12 inch thickness and a central bore 13which is 0.24 inch in diameter. The opening of the receptacle has acounterbore 13a which forms a shoulder 14.

A metal cylindrical closure member 18 (shown in FIGS. 2 and 3) is sizedfor reception in the counterbore 13a and has two passages 19, 19aextending therethrough. The lower end of the member 18 is counterboredto receive a thin, tubular tube 15 constructed of brass which is brazedto the member 18 as shown at numeral 16.

Each of the passages 19, 19a receives a bare wire conductor 11, 11a madeof an iron-cobalt (15-20%) and nickel (25-30%) alloy which ishermetically sealed therein by a glass bead 10a. The beads 10:: areobtained by a well-known technique in electronics, by inserting a littleglass tube into each passage with a conductor and melting the glasswhich by dissolving alloy oxides forms hard welding. While cooling, thepassage 19 hoops the glass bead. Within the tube 15, the ends ofconductors 11, 11a are attached to the ends of a helically wound wireresistor 24 which has an electrical resistance of 50 ohms. Resistor 24lies approximately along the center line of tube 15. The tube 15 isfilled with an alumino-thermic igniting mixture 25, for example, aweakly compressed homogeneous mixture of aluminum and barium peroxidepowder in roughly stoichiometric proportions (Al:20% and BaO A lid 26 isprovided for the lower end of tube 15.

In the bottom of receptacle 10 is a thin-walled container 17 enclosing abooster explosive 28 such as lead nitride. Container 17 is closed at itsupper end by a thinwalled crimped disc 29.

The assembly comprising the tube 15 of aluminothermic powder -and thecover 18 practically occupies all of the free space within thereceptacle 10 above the lead n-i- .tride booster charge 28 and is jammedinto tight contact with the case 17. Cover 18 has an outer recessedsurface 21 to form 'an annular recess with the receptacle 10 whichpermits a metal seal 30 to be made between the cover 18 and thereceptacle 10. Seal 30 is thus a hermetic se-al. Cover 18 is brazed ontoreceptacle 10 by high frequency or induction heating which does notaffect explosive mixture 25 since the explosive mixture requires highertemperatures for detonation.

With such an embodiment the metal seal 30 and the wire-welded glassbeads 110a seal the detonator very tightly so that the assembly is gasproof and fluid tight, even for hydrocarbon gases under very highpressures, since these external high pres-sures are transmitted by themetal seal 30 to the cover body thereby increasing the hooping of theglass beads against the wall of the passages 19 and 19a. In electronics,glass beads and iron cobalt-nickel conductors ensure insulation of thesaid conductors but a high degree of fluid-tightness is neither requirednor obtained. But, by a good choice of the cover shape and the sealposition one obtains an increasing of the bead hooping by the highpressures applied on the detonator, thereby allowing the said detonatorto withstand the said pressures without any drawback.

Furthermore, the steel wall of receptacle has a thickness of ampledimensions to withstand any high pressures.

Operation of the detonator acocrding to the invention at temperatures inexcess of 200 C. is ensured. Indeed, the alumino-thermic mixture isperfectly stable at a temperature up to 677-700 C. and lead nitride 28withstands without detonating, a temperature of 270 C. The safety marginthus is great compared with ambient temperatures encountered in a wellbore.

During the lowering of a perforating apparatus into the well bore, heavyvibrations occur. Because the alumino-thermic mixture 25 and the leadnitride 28 are separately enclosed, abrasive contact of the explosivewith metal parts is prevented. Hence, the detonator is insensitive toany such vibrations.

With regard to stray electrical currents and static charges, theprotection of the detonator according to the invention, is directlyensured by the high resistance of the igniter resistor 24. While in theinstance described, the resistor 24 has a resistance of 50 ohms, itsvalue may vary according to cases, from to 80 ohms. Static voltagestypically found in well bores generate in resistor 24 only currentswhich are too Weak to develop sufiicient temperature to ignite theexplosive mixture 25. The same holds true with respect to stray currentswhich may be induced by adjacent electrical conductors passing thedetonator.

Another advantage of a detonator according to the invention, in thepresence of static charges which have been stored by friction againstthe outer sides of the detonator, results from the alumino-thermicmixture which itself provides a low resistance path to discharge staticcharges as they occur.

If a control voltage of 25 volts is applied from a power source (notshown), a 0.5 amp. current flows through resistor 24. The resistor 24 isheated slowly and after approximately 1 second, its temperature rises tosome 700 C. At the same time, the alumino-thermic mixture 25 and thelead nitride 28 still remain at the pressure conditions of the assemblybecause the thickness of wall 12 of the receptacle prevents the externalpressure from significantly alerting the internal pressure, and, ofcourse, no gas or liquid under high pressure can enter the receptacle.As soon as the critical temperature of 600700 C. for igniting themixture 25 of aluminium and barium peroxide is reached, a reactionoccurs at the surface of the resistor 24. This reaction is quicklytransmitted to the remainder of the mixture 25 releasing a greatquantity of heat energy. This great quantity of heat is confined inreceptacle 10 and detonates the lead nitride mass 28., It is Well-knownthat the barium peroxide BaO is an unstable product which easily becomesbariurn oxide (BaO) and oxygen. This feature explains Why analuminumbarium peroxide mixture reacts from 600700 C. instead of 1000 C.like usual alum-ino thermic mixtures, needing thus a low ignitingcurrent.

Thus, a detonator according to the invention, is capable of operationunder very severe temperature and pressure conditions encountered inwell bores.

It is well understood that the invention is not limited to the examplesdescribed and illustrated above which have solely been used asnon-restrictive examples, but on the contrary the invention may be madein other alternative manners.

Thus, one can use a single head traversed either by both conductors oronly one, as in the case where one end of the igniter is connected tothe jacket or wall of the detonator.

Likewise, the attachment of cover 18 to receptacle 10 may be made inanother way than by brazing, for example, an autogenous soldering or ametal-to-metal seal which can be accomplished by pressure setting orshrinkfitting methods.

As for the composition of the igniting compound, black powder which isstable up to 300 C. can be also used if the detonator is carefully made.With special cables which can withstand large currents, usualalumino-thermic mixtures can be used.

Likewise, the lead nitride 28 may be replaced by another primaryblasting agent, or even by a secondary one, if sufficient quantities,provided the the explosives be stable at temperatures in excess of 200C.

As for the receptacle 10, it can be made of material other than steel,provided its resistance to the pressure is great enough and itsdimensions are not excessive.

Furthermore, instead of being made of an alloy of iron-cobalt andnickel, Wires 11 and 11a can be made of tungsten or molybdenum oxides ofwhich are also dissolved by melting of a suitable glass, and thus form ahard welded bond.

An alternative concerning the receptacle 10 is employed when a detonatoraccording to the invention is used as a detonator for a quick fuse or ashaped charge. In these cases, the bottom of receptacle 10 is machinedfor direct attachment to the tip of said quick fuse or shaped charge.

While particular embodiments of the present invention have been shownand described, it is apparent that changes and modifications may be madeWithout departing from this invention in its broader aspects and,therefore, the aim in the appended claims is to cover all such changesand modifications as fall within the true spirit and scope of thisinvention.

What is claimed is:

1. A detonator comprising: a hollow metal receptacle having an open anda closed end; a booster explosive received in said closed end of saidreceptacle, said explosive being stable up to a temperaturesubstantially not less than 200 C.; and means for detonating saidbooster explosive including an igniting composition separately disposedin said receptacle proximate to said booster explosive, said ignitingcomposition being stable up to a temperature substantially not less than600 C., an electrical resistor disposed Within said ignitingcomposition, said resistor having an electrical conductor leading fromone end thereof, a metal cover having a glass tubular electricalinsulator sealingly engaged therein, said conductor being passed throughsaid tubular insulator and sealingly engaged therein, said cover beingsealingly engaged with said receptacle to hermetically close said openend thereof; said receptacle, cover and insulator being adapted andengaged one to the other to withstand high pressures external thereto,the resistance of said resistor being in the range of 15-80 ohms andcapable of being heated to temperatures in excess of 600 C. inapproximately one second in response to a sustained application ofelectrical current, yet being insensitive to stray electrical currents.

2. A detonator comprising: a hollow metal receptacle having an open anda closed end; a first hollow case received in said closed end of saidreceptacle; a booster explosive disposed in said first hollow case, saidexplosive being stable up to a temperature substantially not less than200 C.; means for detonating said booster explosive including a secondhollow case received in said receptacle and disposed with one endthereof in detonating proximity to said booster explosive, an ignitingcomposition disposed in said hollow case, said igniting compositionbeing stable up to a temperature substantially not less than 600 C., anelectrical resistor disposed within said igniting composition, a metalcover received in said open end, means for conducting current throughsaid resistor including a metal electrical conductor passed through saidmetal cover and attached to said resistor, electrical insulation meanssealing said conductor relative to said metal cover, and means sealingsaid cover relative to said receptacle; the resistance of said resistorbeing in the range of 15-80 ohms and capable of being heated totemperatures in excess of 600 C. in approximately one second in responseto a sustained application of electrical current, yet being insensitiveto stray electrical currents.

3. The detonator of claim 2 wherein said resistor has a resistance of 50ohms.

4. An igniter for a blasting cap including an enclosure member forreceipt within a blasting cap container, an igniting compositiondisposed in said enclosure member, said igniting composition beingstable up to a temperature References Cited by the Examiner UNITEDSTATES PATENTS 991,373 5/11 Rennie et al. 102-28 1,617,675 2/27 Djidics102-28 2,240,438 4/41 Durant 10228 2,882,820 4/59 Young 10228 2,926,5663/60 Atkins et a1 10228 X 2,953,443 9/60 Lloyd 102-28 X 2,987,997 6/61Ireland 10228 3,040,660 6/62 Johnston 10228 3,054,351 9/62 Apodaca etal. l0270 3,059,576 10/62 Haefner 10228 SAMUEL FEINBERG, PrimaryExaminer.

4. AN INITER FOR A BLASTING CAP INCLUDING AN ENCLOSURE MEMBER FORRECEIPT WITHIN A BLASTING CAP CONTAINER, AN IGNITING COMPOSITIONDISPOSED IN SAID ENCLOSURE MEMBER, SAID IGNITING OCMPOSITION BEINGSTABLE UP TO A TEMPERATURE SUBSTANTIALLY NOT LESS THAN 600*C., ANELECTRICAL RESISTOR DISPOSED WITHIN SID IGNITING COMPOSITION, ANELECTRICAL CONDUCTOR MEANS ATTACHED TO SAID RESISTOR FOR CONDUCTINGCURRENT THROUGH SAID RESISTOR, THE RESISTANCE OF SAID RESISTOR BEING INTHE RANGE OF 15-80 OHMS AND CAPABLE OF BEING HEATED TO TEMPERATURES INEXCESS OF 600*C. IN APPROXIMATELY ONE SECOND IN RESPONSE TO A SUBSTAINEDAPPLICATION OF ELECTRICAL CURRENT, EYT BEING INSENSITIVE TO STRAYELECTRICAL CURRENTS.